Minor: “should this be copy?” for NSNumber properties

I haven't quite decided whether this exactly what I want.  On one hand, it is indeed a copy of the internal state that's generated each time it's requested.  On the other hand, maybe this is none of the using class's business— if I decide I want to change the internal storage from a `double` to an `NSNumber *`, I should be able to without touching the h-file.  Perhaps what I really need is an NSNumber sublclass that auto-mirrors the property's state (P001NSNumberPropertySurrogate)…

Regardless, for now I'm just adding this note.

• Added a “should this be copy?” comment to NSNumber properties on models that provide them as a mirror of the internal state (InchCoord).